I know what you are going through, my 79 Pete leaks down at about the same rate as yours. One thing that I did do was to put an extra check valve at the compressor that helped alot. I am in the process of re plumbing all of my air lines they are old and when I sprayed them down with soap some of them slowly formed bubbles like they were seeping air and I had spent hours just soaping the connecters. It can be a long frustrating process,small victories and soon you will be there.

 

wondering if maybe the compressor has a leak

 

Just a thought. My bosses truck did the same thing only in about 30 minutes. Turned out engine fan hub was the problem.

 

OMG Rick...my Freightliners doing the same thing, can hear a hiss under the dash unless I move to re-cycled air, when moved to re-cycled air no hiss....hmmm someone said its out of the heater? Plz e-mail me what your was id you find it!....oopss yours was gone with defrost....not recycled ;-p

 

Finally Found both of my air leaks.

1. The air governor was starting to go bad and leaking down, as well one connection was not tight. This stopped the leak down very much.

2. The other one, I am not sure what it is called, but when I switch from defrost to vent, I hear the door flap shutting, but I hear a real bad hissing sound in the dash. I can switch it back over to defrost and it stops, any suggestions on this?

Remember this is a 1985 FTL, could there be some type of push rod valve that may not be closing?

 

You can say that again!